# Thread: Black holes questions

1. I see on the ol' TeeVee, on the science programs, they occasionally have shows on black holes and there they talk about "super-massive black holes."

My question is, Does a super massive black hole have more or less or equal gravitational pull than a normal sized black hole? Does the "super massive" refer to their size only? (measuring many light years across) Can there be limit in the size of black holes? I'm thinking more on the small side. Can there be a black hole the size of a quarter or a basketball hoop? and if so would it be just as strong as a super massive black hole?
Thanks.

2.

3. Supermassive means very massive. These are found at the center of most galaxies and are of the order of a million or a billion solar masses in size. Ordinary black holes are of the order of a few solar masses in size.

The radius of a mass hole is directly proportional to its mass.
You can find details in
Black hole - Wikipedia, the free encyclopedia.

The largest are estimated to have a radius of ~ 400 A.U. (1 A.U. is earth-sun distance).

4. What about the strength of them?

5. The gravitational force they exert is proportional to the mass.

6. Originally Posted by PhDemon
The gravitational force they exert is proportional to the mass.
I may be older than you by 4 months, but you're smarter than me by light years. Does that mean the bigger they are the stronger their pull is?

I was asking this because I was going to post in the science fiction (or some such similar place) thread, if one day it would be possible that instead of having kitchen trash cans or compactors we may have our own little black holes where we could throw garbage and then we could put the lid back on the trash can and the Black hole would vanish until we take the lid off again to throw something else away.

7. The "pull" depends on how much mass they have, the radius is also proportional to the mass (as mathman said), so yes, the "pull" is proprtional to the radius (but it is the mass that is the fundamental property which is responsible for the other two). As for little blackholes (this is now getting close to the edge of my knowledge and I may be corrected) they aren't stable, micro black holes evaporate by Hawking radiation faster than they can gain new mass by things falling into them, one small enough to be used as a trash compactor would not last long.

8. Originally Posted by PhDemon
The "pull" depends on how much mass they have, the radius is also proportional to the mass (as mathman said), so yes, the "pull" is proprtional to the radius (but it is the mass that is the fundamental property which is responsible for the other two). As for little blackholes (this is now getting close to the edge of my knowledge and I may be corrected) they aren't stable, micro black holes evaporate by Hawking radiation faster than they can gain new mass by things falling into them, one small enough to be used as a trash compactor would not last long.
For now the only way to make a new black hole, is by a star that's many solar sizes larger than our sun going supernova and creating one. I believe in micro sized black holes about as much as I believe in God. They don't exist in nature and I don't believe they can ever be created artificially.

9. But they would result if a larger blackhole having exhausted the supply of matter close enough to fall into it evaporated away via Hawking radiation, they don't have to be formed small...

10. Originally Posted by PhDemon
The "pull" depends on how much mass they have, the radius is also proportional to the mass (as mathman said), so yes, the "pull" is proprtional to the radius (but it is the mass that is the fundamental property which is responsible for the other two).
Thanks

Originally Posted by PhDemon
As for little blackholes (this is now getting close to the edge of my knowledge and I may be corrected) they aren't stable, micro black holes evaporate by Hawking radiation faster than they can gain new mass by things falling into them, one small enough to be used as a trash compactor would not last long.
I saw by the link that mathman posted to the wiki black hole page that said little black holes were/are created by collisions but it didn't say anything about evaporation.

At the risk of taking this in the direction of the further fantastic, the black hole wouldn't have to last long, just long enough to throw trash away and then if it closes on you you just replace the lid and take it off again which gives you a new black hole...
Could you imagine instead of calling the plumber to fix your garbage disposal you'd have to call a physicist.

11. For black hole evaporation see here Hawking radiation - Wikipedia, the free encyclopedia . As for the rest it would make an interesting basis for a Sci fi story

12. Originally Posted by PhDemon
But they would result if a larger blackhole having exhausted the supply of matter close enough to fall into it evaporated away via Hawking radiation, they don't have to be formed small...
Your making one hell of an assumption aren't you? Hawking radiation is still an unproven theory and most likely doesn't exist either IMO. And even if it is true, any solar sized black hole isn't going to evaporate to micro size inside of a 100 billion years of more. So they just don't exist, and from our POV never will.

13. Originally Posted by PhDemon
For black hole evaporation see here Hawking radiation - Wikipedia, the free encyclopedia . As for the rest it would make an interesting basis for a Sci fi story
Thanks for taking me seriously. Further proof you should do well in the classroom, since I suspect my questions are about the caliber of questions you will field in class.

Definitely a decent Sci fi story idea. Especially when future mobsters are stuffing bodies into them... no body no crime, how would you prosecute? now I'm splintering off into the philosophy thread...

14. @Bad Robot. The question was about small black holes, I answered in light of what I'd read about the subject that's all...You may be right but work on what I posted has been published in peer reviewed journals whether you find it believable or not...

15. Originally Posted by PhDemon
@Bad Robot. The question was about small black holes, I answered in light of what I'd read about the subject that's all...You may be right but work on what I posted has been published in peer reviewed journals whether you find it believable or not...
Okay, I'm good with you now.

grmpysmrf

Black holes are one of the main reasons why our universe exist as it does. With out them we wouldn't exist, so be very glad that they do. That extra powerful gravity you mentioned is only that way on the other side of the event horizon. On this side of any event horizon the gravity is just like that of any other celestial bodies in the universe.

16. Originally Posted by Bad Robot
grmpysmrf

Black holes are one of the main reasons why our universe exist as it does. With out them we wouldn't exist, so be very glad that they do.
I love black holes, So full of mystery. I can't wait for science to create an anti gravity suit so we can go spelunk those bad boys... Unfortunately I'll be long dead before that, but if I play my cards right, after I die I'll be cryogenically frozen in the shape of a triangle, and held in a triangular storage device and be one with the universe, therefore already knowing the secrets of the black hole and eventually become my own triangular everything!!!

Originally Posted by Bad Robot
That extra powerful gravity you mentioned is only that way on the other side of the event horizon. On this side of any event horizon the gravity is just like that of any other celestial bodies in the universe.
Weird how it's almost literally a line in the sand. I'd think the pull would intensify as you got closer to the event horizon. Odd that it's "normal... normal... normal... oop, you're F*cked!"

17. The pull does get stronger as you approach (as it does for everything else) the difference is that a black hole is dense enough and has enough mass that there is a point where the "pull" is strong enough that not even light can escape. That's the line in the sand...

18. Originally Posted by PhDemon
The pull does get stronger as you approach (as it does for everything else) the difference is that a black hole is dense enough and has enough mass that there is a point where the "pull" is strong enough that not even light can escape. That's the line in the sand...
Is it actually a hole the way we understand a hole? or maybe just a sticky spot in space where stuff gets trapped (fly paper). For example, if we were able to generate an anti gravity suit would there be a hole to navigate through/explore or would it just be regular space and with nothing different about it since we were wearing the suit? Thanks for entertaining this as long as you both have.

Wouldn't it be neat if black holes were not natural but created by some ancient long dead advanced species as kinda like land mines in some intergalactic war? Man, I love science fiction!!

19. As far as I know, nobody knows, as nothing can escape from a black hole anyone who says anything about what is beyond the event horizon is speculating...(maybe Markus or someone who knows more than me about thus could comment).

20. Originally Posted by PhDemon
As far as I know, nobody knows, as nothing can escape from a black hole anyone who says anything about what is beyond the event horizon is speculating...(maybe Markus or someone who knows more than me about thus could comment).
I know that, but I thought the math would tell us one way or another, just like the math predicts them. Thanks for your comments.

21. That's why I said maybe Markus could comment, he's the go to guy for the maths of GR...

22. Originally Posted by grmpysmrf
Weird how it's almost literally a line in the sand. I'd think the pull would intensify as you got closer to the event horizon. Odd that it's "normal... normal... normal... oop, you're F*cked!"
The event horizon is the point at which light cannot escape from the black hole, so it is a line of sorts. Other than that the gravity operates the same as any other body of mass in the universe and that's as normal as it gets.

23. Originally Posted by grmpysmrf
I know that, but I thought the math would tell us one way or another, just like the math predicts them. Thanks for your comments.
At present no "maths" exists that can definitively answer this question. In the context of our current theory of gravity ( General Relativity ), space-time remains smooth and continuous across the event horizon and all the way down to the singularity - so you just fall until the tidal forces rip you to pieces, simple as that. The problem with this is that GR is a purely classical theory and does not account for any quantum effects - to tell what really happens beyond the event horizon will require a full model of quantum gravity, which we quite simply don't have yet. That's an area of very active research at the moment.

24. Originally Posted by PhDemon
That's why I said maybe Markus could comment...
Sorry if my "thanks for your comment" seemed snide. I truly meant it.

25. Originally Posted by Markus Hanke
At present no "maths" exists that can definitively answer this question. In the context of our current theory of gravity ( General Relativity ), space-time remains smooth and continuous across the event horizon and all the way down to the singularity -
Originally Posted by Markus Hanke
so you just fall until the tidal forces rip you to pieces, simple as that.
Maybe I'm misunderstanding terminology here, but "smooth an continuous" sounds like (for lack of better terms) flat space. but then you said you would "fall" which suggests a hole. Is the idea, a person would still feel like they were in normal space until they were reduced to singularity? feels flat but you're really falling?

For examples, kinda like being pulled from the edge of your living room to the center of it and when you get to the center is when you've reached singularity? so not necessarily a hole but still same outcome? OR would it be your living room is a funnel and you're being pulled to the center of it and really drops you in a hole?

(sorry for asking more specifics when you clearly said on the onset that we just don't have the math for it- I suppose I'm looking for your opinion at this point.)

Originally Posted by Markus Hanke
...to tell what really happens beyond the event horizon will require a full model of quantum gravity, which we quite simply don't have yet. That's an area of very active research at the moment.
Active for you or the community as a whole? how close are you? Is that something the LHC can help with or is that a different area of physics? even if you're still far away from a full model would/could you care to speculate?
Thanks, Mr.(Dr.?) Hanke

26. Sorry if my "thanks for your comment" seemed snide. I truly meant it
No problem, I didn't take it that way I just didn't want to tell you something wrong...

27. He stepped across the kitchen and opened the trash can lid 'zwoosh !'... and was never seen again.. ~ Oops.
No.. I do not think that sort of trash can can be contrived as real. But I do have a related question re; This subject.

" From a stellar mass object I understand that as it's energy exceeds fuel available and the state changes Depending upon the mass available.. Red dwarf, Star, Neutron Star, Quasar, Pulsar, Black Hole, Super massive Black Hole. and that that list could be expanded is the Black hole or the S M B H the final step.. Can I say a Galaxy class SMBH or does it get up to Universe size ( Mass ) BH.." ? Hmm..
~ and no i am not drunk or have even yet had a drink...

28. Originally Posted by astromark
He stepped across the kitchen and opened the trash can lid 'zwoosh !'... and was never seen again.. ~ Oops.
No.. I do not think that sort of trash can can be contrived as real. But I do have a related question re; This subject.

" From a stellar mass object I understand that as it's energy exceeds fuel available and the state changes Depending upon the mass available.. Red dwarf, Star, Neutron Star, Quasar, Pulsar, Black Hole, Super massive Black Hole. and that that list could be expanded is the Black hole or the S M B H the final step.. Can I say a Galaxy class SMBH or does it get up to Universe size ( Mass ) BH.." ? Hmm..
~ and no i am not drunk or have even yet had a drink...
Are you asking if there is any limit to the size of a BH? Damned if anyone knows. But the mass of our universe did come from somewhere. However, the current model of our universe doesn't explain where the mass came from, and I'm not a big fan of "it came from nothing", so where does that leave us?

29. Originally Posted by Bad Robot
Originally Posted by PhDemon
But they would result if a larger blackhole having exhausted the supply of matter close enough to fall into it evaporated away via Hawking radiation, they don't have to be formed small...
Your making one hell of an assumption aren't you? Hawking radiation is still an unproven theory and most likely doesn't exist either IMO. And even if it is true, any solar sized black hole isn't going to evaporate to micro size inside of a 100 billion years of more. So they just don't exist, and from our POV never will.
Actually, Hawking radiation might just be the reason we don't see small black holes. In the seventies Hawking proposed that when the universe was very young, conditions could have been right to form small "primordial" black holes. Since at the time black holes were thought to be perpetual, they should have still existed in our time. Not much later, while considering black holes from another angle, he came up with Hawking radiation. Since the evaporation rate is inversely proportional to the mass, this meant that the majority of primordial black holes formed in the early universe would have evaporated by now.

There still is a chance for some less than stellar mass BH's to exist. Even the most isolated BH is subject to the CMBR. It works out that mass boundary at which point the CMBR absorbed by a BH balances out the loss from Hawking radiation is approximately 1 lunar mass. Any BH larger than this would grow and not shrink. Even one that started out smaller than this could still be around if its evaporation time was long enough. It depends on whether or not primordial black holes were ever formed and if they did, were any of them massive enough to have survived since then.

30. Originally Posted by Janus
Originally Posted by Bad Robot
Originally Posted by PhDemon
But they would result if a larger blackhole having exhausted the supply of matter close enough to fall into it evaporated away via Hawking radiation, they don't have to be formed small...
Your making one hell of an assumption aren't you? Hawking radiation is still an unproven theory and most likely doesn't exist either IMO. And even if it is true, any solar sized black hole isn't going to evaporate to micro size inside of a 100 billion years of more. So they just don't exist, and from our POV never will.
Actually, Hawking radiation might just be the reason we don't see small black holes. In the seventies Hawking proposed that when the universe was very young, conditions could have been right to form small "primordial" black holes. Since at the time black holes were thought to be perpetual, they should have still existed in our time. Not much later, while considering black holes from another angle, he came up with Hawking radiation. Since the evaporation rate is inversely proportional to the mass, this meant that the majority of primordial black holes formed in the early universe would have evaporated by now.

There still is a chance for some less than stellar mass BH's to exist. Even the most isolated BH is subject to the CMBR. It works out that mass boundary at which point the CMBR absorbed by a BH balances out the loss from Hawking radiation is approximately 1 lunar mass. Any BH larger than this would grow and not shrink. Even one that started out smaller than this could still be around if its evaporation time was long enough. It depends on whether or not primordial black holes were ever formed and if they did, were any of them massive enough to have survived since then.
Those are very good points. But Hawking radiation depends on the fabric of space/time being composed of virtual particles that come into being in pairs of particle & anti-particle that normally annihilate each other in a time so short that it remains undetectable to us. When these particles pop into existence very near an event horizon the theory says that when the particle of the pair escapes and the anti-particle is captured. The escaped particle becomes Hawking radiation and the captured anti-particle reduces the mass of the BH by the same amount as the escaped particle. In other words not very much. If this is even remotely true? What prevents the same number of anti-particles from escaping and thus balancing out the mass exchange?

31. Originally Posted by Bad Robot

Those are very good points. But Hawking radiation depends on the fabric of space/time being composed of virtual particles that come into being in pairs of particle & anti-particle that normally annihilate each other in a time so short that it remains undetectable to us. When these particles pop into existence very near an event horizon the theory says that when the particle of the pair escapes and the anti-particle is captured. The escaped particle becomes Hawking radiation and the captured anti-particle reduces the mass of the BH by the same amount as the escaped particle. In other words not very much. If this is even remotely true? What prevents the same number of anti-particles from escaping and thus balancing out the mass exchange?
It doesn't matter whether it is the mater or antimatter particle of the pair that escapes (both matter and antimatter have positive mass). It's the fact that either escapes. The energy books must remain balanced. When a virtual particle pair form it is by "borrowed" energy. As long as the debt is payed back before nature misses it, everything is fine. When one particle falls into the BH, the pair cannot recombine and pay off the energy deficit, the black hole ends up paying the debt off that allows for the existence of the particle outside the event horizon.

32. Originally Posted by Janus
Originally Posted by Bad Robot

Those are very good points. But Hawking radiation depends on the fabric of space/time being composed of virtual particles that come into being in pairs of particle & anti-particle that normally annihilate each other in a time so short that it remains undetectable to us. When these particles pop into existence very near an event horizon the theory says that when the particle of the pair escapes and the anti-particle is captured. The escaped particle becomes Hawking radiation and the captured anti-particle reduces the mass of the BH by the same amount as the escaped particle. In other words not very much. If this is even remotely true? What prevents the same number of anti-particles from escaping and thus balancing out the mass exchange?
It doesn't matter whether it is the mater or antimatter particle of the pair that escapes (both matter and antimatter have positive mass). It's the fact that either escapes. The energy books must remain balanced. When a virtual particle pair form it is by "borrowed" energy. As long as the debt is payed back before nature misses it, everything is fine. When one particle falls into the BH, the pair cannot recombine and pay off the energy deficit, the black hole ends up paying the debt off that allows for the existence of the particle outside the event horizon.
Be that as it may, who ever came up with this virtual particle theory anyway? Where is there any proof that it is even close to reality. As far as I know it has never been demonstrated that virtual particles even exist. If our space/time is really made up of virtual particles that would certainly change much about how we all think about reality. For instance where would all these virtual particles be coming from? When they annihilate each other, what happens to the energy? What problems does this theory actually fix with our current theories of our universe?

33. It's a consequence of Heisenbergs Uncertainty Principle (Virtual Particles and Heisenberg's uncertainty principle) a key part of quantum mechanics, a theory that has been very well tested and experimentally verified...(for example http://physics.about.com/od/quantump...imirEffect.htm )

34. Originally Posted by PhDemon
It's a consequence of Heisenbergs Uncertainty Principle (Virtual Particles and Heisenberg's uncertainty principle) a key part of quantum mechanics, a theory that has been very well tested and experimentally verified...(for example What is the Casimir Effect? )
Demonstrating the Casimir Effect might suggest virtual particles, but I'd like to see some additional proof. Has anybody tied Dark Energy with virtual particles yet? Just thinking that might solve the problem of our expanding universe.

35. So you're ignoring the uncertainty principle (pretty much fundamental to QM that works) and think the Casimir effect (a real experimentally verifiable effect that is strong evidence these particles exist) is not good enough and still asking for evidence? I don't know what more you want :shrug:

36. I might be very alone with this; Does anyone else on hearing the words 'Uncertainty principal'
~ Immediately see the Hart of Gold and Ziphod looking at a tea bag..
I am encouraged to share and do understand not to have helped.. just thought you should know.

37. Originally Posted by astromark
I might be very alone with this; Does anyone else on hearing the words 'Uncertainty principal'
~ Immediately see the Hart of Gold and Ziphod looking at a tea bag..
I am encouraged to share and do understand not to have helped.. just thought you should know.

38. you won't feel the pull until you are very close to them, till your body begin to feel the gravitational difference

39. Originally Posted by grmpysmrf
I see on the ol' TeeVee, on the science programs, they occasionally have shows on black holes and there they talk about "super-massive black holes."

My question is, Does a super massive black hole have more or less or equal gravitational pull than a normal sized black hole? Does the "super massive" refer to their size only? (measuring many light years across) Can there be limit in the size of black holes? I'm thinking more on the small side. Can there be a black hole the size of a quarter or a basketball hoop? and if so would it be just as strong as a super massive black hole?
Thanks.
Supermassive - it's not the size, it's weight.
Size of black holes is not known.
There is the event horizon of a black hole.

40. Originally Posted by smartkensol

Supermassive - it's not the size, it's weight.
Size of black holes is not known.
There is the event horizon of a black hole.
The term you are looking for is mass (not weight). For instance your weight would change depending on the gravity of the world you are on. But your mass will be the same.

For any black hole more mass equals more gravity and a corresponding greater diameter event horizon. What determines the event horizon is that is the point at where light can no longer escape the gravity of the black hole.

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